BIOL 2500 - Topic 7

Every species on Earth varies in…

1.) Chromosome structure

2.) Chromosome number in the nucleus

Chromosome structure

It follows a specific pattern of sizes and shapes, with there being 3-4 general chromosome shapes found in most species

Chromosome organization in the nucleus

Within the nucleus, chromosomes occupy territories, such that if you are looking for chromosome 2, you can always find it in the territory it is in

Chromosomal territories

Each territory contains a single chromosome within an arbitrary region that is not bound by any membrane

Karyotype

A visual image of all the chromosomes, from biggest to smallest, which are fluorescently labelled to help us see the bands

General number of chromosomes

1.) Chromosomes 1-22 are autosomes

2.) Chromosome 23 is a sex chromosome

Different types of chromosomes

There are four different chromosome types, based on the position of the centromere and the length of the chromosome arms

Chromosome arms are usually ________ in length

Unequal

Chromosome arms

Short arm (p)

Long arm (q)

4 different types of chromosome shapes

1.) Metacentric chromosomes

2.) Submetacentric chromosomes

3.) Acrocentric chromosomes

4.) Telocentric chromosomes

Metacentric chromosomes

The centromere is near the middle, therefore the p arm = q arm

Submetacentric chromosomes

The centromere is slightly nearer to one end, therefore p arm < q arm

Acrocentric chromosomes

The centromeres are at the very end, therefore p arm (very) << q arm

Telocentric chromosomes

They have a terminal centromere, therefore they only have q no p

Most common types of chromosomes

Submetacentric and acrocentric chromosomes

Visualizing chromosomes

As chromosomes condense for cell division, we can use various staining and fluorescent dyes to highlight regions on the chromosomes, allowing us to see the banding patterns

Significance of banding pattern

It helps ID chromosomes, as each chromosome has a specific banding pattern

What causes the banding patterns we see?

Chromatin condensation

Euchromatin

1.) Regions that are less compact, usually those being actively transcribed

2.) Some are always in this form, such as rRNA genes

Heterochromatin

1.) Regions that are highly compact, resulting in less gene transcription

2.) Some are always in this form, such as Barr bodies

Can heterochromatin be found in G1 and G2?

Yes, specifically at genes that are not being transcribed

Chromosomal mutations

Large-scale changes in chromosomes, different from DNA/point mutations

Chromosomal mutations vs. point mutations

Chromosomal mutations are much larger than point mutations as it affects multiple genes, not just one, resulting in major phenotypic changes, as it causes changes in chromosome numbers and/or structure

Detection of chromosomal mutations

It can be detected under a microscope by seeing differences in the banding pattern (which you cannot do with point mutations)

Two types of changes in chromosome numbers

1.) Aberrant euploidy —> changes in whole chromosomal sets (2N —> 4N)

2.) Aneuploidy —> Changes in parts of chromosomal sets (Loss or gain of a chromosome #2)

Euploids

Organisms with multiple of the basic chromosome set, such as diploid, triploid, etc

Normal euploid

Diploid (normal number of sets)

Aberrant euploid

Organisms that have more or less than the regular euploid number (i.e. ants, wasps, strawberries, etc)

Abberant euploids examples

1.) Monoploid

2.) Triploid (3n)

3.) Tetraploid (4n)

Monoploid vs. haploid

Haploid refers to organisms that usually only have one set of chromosomes, whereas monoploids refer to organisms that usually have 2 sets of chromosomes but only have one

Polyploidy

Organisms with more than 2 chromosomal sets

Origin of polyploidy

Some can be a result of aberrant euploidy, but some are just naturally polyploid, such as most plants

The more the chromosomal sets, the ______ the organism

Bigger

Even vs. odd sets of chromosomes

Even chromosomal sets are much more common than odd sets, as whole genomes usually double and redouble

Autopolyploidy

Polyploidy where the chromosomes are derived from a single species

Allopolyploidy

Polyploidy where the chromosomes are derived from multiple species

How triploids arise

They are usually autopolyploid, by spontaneous nondisjunction

Pairing of triploids

1.) Because there are uneven numbers of chromosomes, triploids cannot pair during meiosis

2.) Therefore, the bivalents separate as expected, but univariants randomly go to either pole

Resulting number of chromosomes in gametes in triploids

1.) One gamete has 2 chromosomes (n+1)

2.) The other has 1 chromosome (n)

Autotetraploids

They arise from the doubling of a 2N set to a 4N set, occurring either spontaneously or by chemical disruption (such as colchicine, which induces autotetraploidy)

Autotetraploids during meiosis

Unlike triploids, they can undergo regular meiosis where the chromosomes can pair up since we have an even number of chromosomes, resulting in an even reduction division

Paring possibilities of autotetraploids

1.) Two bivalents

2.) One quadrivalent

3.) A univalent + a trivalent (but this is rare)

Example of triploids (plants)

Bananas, which are sterile and can only reproduce via induced asexual reproduction

Food crops that are polyploid plants

1.) Bananas

2.) Strawberries

3.) Wheat

Consequences of polyploid plants

1.) Fruit and flower size increase

2.) But have reduced fertility (i.e. results in seedless fruits)

3.) Also observe an increase in heterozygotes relative to diploids due to hybrid vigor

Hybrid vigor

It is when hybrids are more fit and grow faster/bigger than true-breeding plants, which is why we see an increase in heterozygotes relative to diploids

Allopolyploids

It is caused by the hybridization of two 2 or more species coming together to form a new species

Allopolyploids are created by…

Amphidiploids

Amphidiploids

*ASK J*

Can hybrids cross with any of the species from which it arose from?

No, because it would have chromosomes that the other species does not

Aneuploidy

A change in the number of one of the chromosomes, caused by nondisjunction (failure to separate)

Examples of aneuploidy

1.) Monosomic

2.) Trisomic

3.) Nullisomic

Monosomic

2n - 1 (missing one chromosome)

Trisomic

2n + 1 (have an extra chromosome)

Nullisomic

Missing the homologous chromosomes entirely (BBCC, instead of AABBCC)

Nondisjunction

When homologous chromosomes and sister chromatids fail to separate properly

Effect of nondisjunction

1.) It affects the number of chromosomes within the cells, such that some gametes have more copies while the others have less copies

2.) This then alters the phenotype, which can affect development and/or fertility

Nondisjunction in somatic cells vs. germline cells

It can occur in both, but it can only be inherited when it occurs in germline cells

Nondisjunction in germline cells causes…

Aneuploidy

Nondisjunction in meiosis I and meiosis II

It only occurs once throughout meiosis, such that if it occurs in meiosis I, then meiosis II will proceed normally and vice versa

Nondisjunction in meiosis I

It results in the failure of homologous chromosomes to separate; therefore, after meiosis I, one of the daughter cells has both chromosomes while the other has none

Resulting gametes when nondisjunction occurs in meiosis I

½ of gametes will be n + 1 (trisomic)

½ of gametes will be n -1 (monosomic)

Nondisjunction in meiosis II

It results in the failure of sister chromatids to separate, therefore nondisjunction only occurs in one of the two daughter cells after meiosis I

Resulting gametes when nondisjunction occurs in meiosis II

½ will be n (euploid)

¼ will be n-1 (null?)

¼ will be n+1

Monosomics

Missing one copy of a specific chromosome, while all the other chromosomes are considered normal (2n-1)

Monosomics in animals

When it occurs in autosomes, it often leads to abortions/miscarriages, but it can be somewhat tolerated in sex chromosomes

Monosomics in a sex chromosome example

Turner syndrome (one X instead of two X’s)

Why is X0 significant if one of the X’s is inactivated anyway?

2 chromosomes are still needed for normal development in females, as the genes on a single X chromosome are not enough for development

Can turner syndrome occur in males

Yes, as a result of missing parts of the Y chromosome, but it is rare

How does turner syndrome occur

It occurs when one of the gametes has an n-1

Trisomics

Having an extra copy of a specific chromosome (2n + 1)

Trisomy at different chromosomes

There are potentially 24 different kinds of trisomy, but mostly in 13, 18, 21, and the sex chromosomes

Trisomy vs. monosomy (throughout life)

Trisomy can occur in autosomes and still be passed on, such that some individuals are able to grow and reach adulthood, which does not occur in monosomy

Trisomics examples

1.) Triple X syndrome (XXX)

2.) Klinefelter syndrome (XXY)

Can XXY individuals produce gametes?

They they can, but they may not be viable

Trisomy 21 (autosomal or sex-chromosome?)

Autosomal but… *ASK J*

Trisomy 21

Aka Down syndrome, which is the most common autosomal aneuploidy in humans

Cause of trisomy 21

90% is a result of nondisjunction during meiosis I, 10% occur via Robertsonian translocation

Chromosome distribution in trisomy 21

They have 2 maternal and 1 paternal copy of chromosome 21

Trisomy 21 is linked to…

1.) It is linked to the age of the mother, as a result of the pause that occurs during prophase I

2.) The older you get, the longer the pause, the more likely nondisjunction occurs

When does the pause in prophase I stop?

Monthly cycles reinitiate meiosis, allowing eggs to be released into the fallopian tubes after meiosis I, but meiosis II won’t begin until after fertilization with a sperm

Blakeslee and Belling

They worked on Jimson weed and recognized that aneuploidy alters gene dosage, resulting in a variety of phenotypes

Gene dosage effect

Refers to the fact that aneuploidy changes the gene dosage of all the genes on the affected chromosomes, therefore leading to an imbalance in gene products

Normal gene dosage in diploid organisms

100% (anything less or more leads to phenotypic differences)

Gene dosage in monosomic vs. trisomic individuals

1.) Monosomic (2n-1) = 50%

2.) Trisomic (2n+1) = 150%

Effect of gene dosage in animals vs. plants

Animals are usually more affected by it, while plants are more tolerant of the changes it causes, as they have different developmental programmings than animals

Effect of gene dosage on animals

Most of the time it is lethal, as it causes developmental delays and/or nervous system underdevelopment

X-inactivation mosaicism

It is when one X chromosome in female somatic cells randomly inactivates, causing different cells to have slightly different phenotypes

Can nondisjunction happen in mitosis

Yes, but it only affects the embryos body, as it occurs during embryogenesis early in development, and does not get passed down to future progeny

X-inactivation mosaic Turner syndrome

25-30% of cases regarding turner syndrome can result in a mosaic of XO monosomy (C45), while the rest are XX (C46), but some individuals also have XXX cells (C47)

Uniparental disomy

It is when both copies of the homologous chromosome pair are derived from the same parent

Uniparental disomy can cause…

1.) Angelman syndrome —> has paternal copies of chromosome 15

2.) Prader-Willi syndrome —> has maternal copies of chromosome 15

Rare cause of uniparental disomy

Nondisjunction occurs at the same chromosome in both the egg and sperm, where females result in XX and the male have 00 (but this is super rare)

Normal cause of uniparental disomy

1.) Nondisjunction occurs in one parent that results in an aneuploid gamete that has two copies of the chromosome and then it gets fertilized by a normal parental gamete

2.) Trisomy rescue occurs, but sometimes it ends up tossing out the normal gamete chromosome instead of the extra chromosome, resulting in the two copies of the chromosome from the same parent

Trisomy rescue

It is when the extra copy of a chromosome is tossed out during embryonic development

Rearrangements

Changes to chromosome structure that can result in the loss or addition of genes

Effect of rearrangements

It causes gene dosage imbalances, which can greatly affect animals

Chromosomal ___________ are very common

Duplications

Significance of chromosomal duplications

It is how we get novel genes (such as hemoglobin)

Importance of chromosomal reaarangements

1.) Source of genetic variation (can even cause speciation)

2.) A major cause of genetic disorders and conditions

3.) It can cause infertility